The invention relates to a system and method for automatically calibrating beamforming coefficients for wireless communication networks. More particularly, the invention relates to a system and method for automatically calibrating beamforming coefficients for wireless communication networks that employ a multiple number of antennas and RF units to perform spatial diversity.
Point-to-multi point communication networks typically consist of a central base station (hub) and several remote users, located at different locations. The hub typically contains an antenna array. Antenna arrays have been used to increase the information capacity sent to remote users. A typical antenna array is capable of forming a beam towards each desired user and placing nulls in the direction of interferers, thus minimizing the total energy required for acceptable link performance.
Each antenna on the antenna array typically has a corresponding receive channel which includes a corresponding receiver and a corresponding channel unit. The typical RF receiver obtains the desired signal by filtering out the unwanted portions of the received broadcast signal. Each RF receiver provides the desired frequency signal to a corresponding channel unit, where the desired frequency signal is digitized in an A/D converter, demodulated in a demodulator and decoded in a decoder. The output of the plurality of channel units are then operated on by a beamforming vector and combined into one signal.
The beamforming operation is typically accomplished by applying an adaptive algorithm to the output of each channel. Under ideal conditions the output of the each of the receive channels produce the same amplitude and quadrature of a receive signal as the other channels, when the signal arrives simultaneously at all antennas, thus the receive channels are said to be balanced. However, due to variations in the respective signal paths between the antennas of the antenna array and the respective corresponding circuitry in the channels, which may be caused by a variety of factors such as manufacturing variances, the channels are not likely to be balanced, and thus the output of the receive channel units may be different in amplitude, phase and quadrature. Accordingly, the beamforming operation should also account for an imperfectly balanced set of receive channels. More particularly, the beamforming operation typically is performed by a predetermined beamforming vector which includes offsets to balance the amplitude, phase and quadrature between the respective channels.
Also in the typical prior art system, the hub is capable of simultaneously broadcasting to a plurality of remote users with a plurality of channels, which is known as downlink. A transmission channel is similar to the receive channel, except in the direction of the signal flow. Thus a transmission channel typically includes a transmission channel unit, a transceiver and an associated antenna. The transmission channel unit typically contains an encoder which encodes the signal, a modulator which modulates the signal and a D/A converter which converts the signal to analog. A beamforming operation is performed to generate a signal into a plurality of signals, each of which is carried and broadcasted by an individual transmission channel. The transmission beamforming operation is similar to the reception beamforming operation, and thus it also accounts for unbalanced transmission channels.
Nulls are also typically formed by the antenna array to minimize interference to the intended remote users. However, unmatched channels can destroy otherwise correctly formed beams and nulls. Unmatched channels may be caused by the inability of the beamforming vector to include the channel in the beamforming operation due to not having the offsets necessary to balance the channel with the other channels. A solution to this problem is to calibrate the transmitting antenna array by radiating test signals to different directions. However, this is a very time-consuming and expensive process. Another solution is to impose a very tight specification for amplitude and phase matching among channels. However, this is also an expensive design solution.
It is an object of the invention to develop an inexpensive, cost effective automatic array calibration scheme which solves the foregoing problems.
It is another object of the invention to develop an automatic array calibration scheme which adjusts to varying transceiver conditions.
It is another object of the invention to develop an automatic array calibration scheme which contains the processing operations of the calibration scheme in an indoor unit of the antenna array system.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, one embodiment of the present invention comprises: a calibration signal generating unit which generates a calibration signal; a calibration signal injecting unit which is operable to selectively inject the calibration signal as an input signal; a receiver unit which receives the input signal and obtains a channel signal from the input signal; a reception data channel unit which receives the channel signal and generates a first set of I/Q signals; and a controller which controls the calibration signal injecting unit to selectively insert the calibration signal as the input signal, and receives the first set of I/Q signals from the reception data channel unit, wherein the controller calculates a reception data channel offset based on the first set of I/Q signals.
In accordance with the objects and purposes of the present invention, in one embodiment the antenna array calibration apparatus further comprises: a transmission data channel unit which includes a transmission data channel being provided with a test signal; a transmission unit which generates a transmission signal based on the transmission data channel; a loop back unit which selectively provides the transmission signal as the input signal, wherein the controller controls the loop back unit to selectively provide the transmission signal as the input signal, the reception data channel unit generates a second set of I/Q signals based on the transmission signal provided as the input signal, and provides the second set of I/Q signals to the controller, and the controller calculates a loop back offset based on the second set of I/Q signals.
In another aspect of the invention, the controller calculates a transmission channel offset according to the equation: Ctx=Cloop/Crx, where: Ctx represents the transmission data channel offset, Cloop represent the loop back offset, and Crx represents the reception data channel offset.
In yet another aspect of the invention, the controller calculates a modification vector according to the equation: Wmod=conj(Cloop)/abs(Ctx)2 where: Wmod represents the modification vector.
In still another aspect of the invention, the controller calculates a transmission beamforming vector according to the equation: Wtx=Wmod*Wrx Where: Wtx represents the transmission beamforming vector, and Wrx represents a reception beamforming vector.
In yet another aspect of the invention, the transmission unit comprises a plurality of transmission data channels, and each channel of the plurality of transmission data channels is simultaneously selectively injected with a test signal when the loop back unit selectively provides the transmission signal as the input signal.
The present invention also relates to a method for obtaining calibration parameters, in one embodiment, the method comprises the steps of: generating a calibration signal as an input signal; selectively injecting the input signal into receive channels; obtaining a first set of I/Q signals; and calculating a reception data channel offset based on the first set of I/Q signals.
The novel method may further comprise the steps of providing a test signal in a transmission data channel; generating a transmission signal from the transmission data channel; selectively providing the transmission signal as the input signal when the calibration signal is not provided as the input signal; generating a second set of I/Q signals, the second set of I/Q signals based on the transmission signal provided as the input signal; and calculating a loop back offset based on the second set of I/Q signals.
The present invention has several advantages over the prior art. These advantages include, but are not limited to: being able to adaptively calculate the transmission beamforming vector by using the receive beamforming vector; calculating the beamforming vector by using internally generated calibration signals, thus eliminating the need to rely on external devices; adaptively calculating offsets without interfering with the operation of the antenna array; inexpensively achieving an offset calibration and transmission beamforming calculation process; and achieving an offset calibration and transmission beamforming calculation process in an indoor unit, thus reducing the cost and increasing durability of the system.
Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention. The invention itself, together with further objects and advantages, can better be understood by reference to the following detailed description and the accompanying drawings.